1. Field of the Invention
The present invention relates to tank venting valves for relieving the internal pressure in tanks at a predetermined pressure. More specifically the invention is concerned with whether a venting valve has opened and actuated.
2. Related Art
There are several types of pressure relief valves for atmospheric or low pressure storage tanks. One of the basic designs is known as a "weight loaded tank vent". The design utilizes a weighted cover over a nozzle for the seating area. The cover, also known as a pallet, forms a barrier to the nozzle opening. The weight of the pallet determines at what pressure the pallet will lift open to relieve pressure in the tank. The start to open pressure indicated by lift of the cover is known as the "set pressure". The set pressure can easily be determined by dividing the weight of the pallet assembly by the open area of the nozzle. If a higher set pressure is desired more weight is added to the pallet.
Other types of pressure relieving devices include a spring loaded tank vent and a pilot operated tank vent. The spring loaded type is used for set pressures above that which is practical for weight loaded operation due to either a high set pressure where there is not enough room for the weights on the pallet, or where the size of the vent nozzle dictates too large of an overall weight on the pallet.
Pilot operated tank vents utilize the tank pressure acting on an area larger than the nozzle area to create a higher downward force to hold down a pallet or seat plate against the tank pressure. The set pressure is adjustable by means of a spring in the pilot.
Any of the three types may be vented to the atmosphere or piped into headers for vapor recovery or burning. Generally these valves open, make an emission or release and close without producing any record of opening and can present a source of atmospheric contamination. Often it is desirable to have a record of when a particular vent valve has opened.
U.S. Pat. No. 5,497,725 discloses a sight indicator which mechanically indicates whether a tank vent valve has opened and also to what approximate degree. The sight indicator has the drawback that a person must actually see the indicator which may require a person to climb on the tank. In addition, the sight indicator will not indicate how many times the vent has opened.
Proximity switches are devices which react to motion without being in actual contact with the moving part. common proximity switches utilize a generated magnetic field that is disturbed when a metallic object moves into the field. These proximity switches are not very sensitive and require substantial movement before detection. Such proximity switches are variously described in U.S. Pat. Nos. 4,618,823; 4,757,213; 4,879,531; 5,012,206 and 5,264,733.
Such a magnetic induction proximity is described generally in U.S. Pat. No. 4,587,486 discloses a magnetic-field-dependent proximity switch which detects the movement of a magnetic field provided by a permanent magnet attached to the moving part. Werner Turck GmbH & Co. manufactures a magnetic induction proximity switch which operates by sensing the movement of a magnetic field which it designates as cylinder position indicators for use on hydraulic cylinders to determine the movement of the cylinder. It is advisable that these devices be potted (hermetically sealed) and have FM or UL type approvals for safety and commercial uses.
It is an advantage that the present device will not interfere with the breather vents function or relief capacities. It is a feature that the present device provides a signal that may be transmitted to a remote location and recorded after the vent valve is closed. It is another feature that the present device can be retrofitted easily to existing vent valves. Additionally, it is a feature that the sensitivity of the device detects movement of less that 1/32 inch so that minor openings of the tank valve can be detected. These and other features and advantages will become apparent from the following descriptions.